This invention relates to a rotational speed control circuit for a motor which is adapted to restrain generation of switching noise and vibration during low-speed rotation of a motor of which a rotational speed is varied, and more particularly to a rotational speed control circuit for a fan motor used for a fan.
A variety of office automation (OA) equipments such as a personal computer, a copying machine and the like each typically have a fan for cooling incorporated therein. Such a fan is driven by a motor of which rotational speed is controlled so as to be increased to rapidly reduce a temperature of the equipment when the equipment is increased in temperature and decreased when it is reduced in temperature. Rotation of the motor at a high rotational speed causes sound generated from the fan being driven such as blowing sound or air cutting sound to be increased to a degree sufficient to drown so-called switching noise or electromagnetic sound generated by a semiconductor switch for controlling an excitation current when the switch is turned off. The semiconductor switch is adapted to control an excitation current flowing through excitation windings of the motor and may be typically constituted by a transistor switch. However, rotation of the motor at a low speed due to a reduction in rotational speed thereof leads to a reduction in blowing sound of the fan, to thereby fail to drown the switching noise or electromagnetic sound, resulting in giving an unpleasant feeling to one. Also, it provides one with an unpleasant feeling due to vibration of the motor derived by the switching.
In order to eliminate such problems, a circuit was proposed which is so constructed that a capacitor is kept constantly connected to a collector-emitter circuit of a transistor switch constituting the excitation current control semiconductor switch.
It was found that such arrangement-of the capacitor restrains generation of switching noise and vibration in a low-speed rotation region of the motor to a certain degree. However, it causes an increase in loss in the transistor switch in a high-speed rotation region of the motor. Also, it causes an increase in excitation current, leading to an increase in generation of heat from the excitation windings.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a rotational speed control circuit for a motor or a fan motor which is capable of restraining generation of switching noise and vibration during low-speed rotation of the motor without affecting high-speed rotation of the motor.
It is another object of the present invention to provide a rotational speed control circuit for a motor which is capable of increasing a period of time for which an excitation current control semiconductor switch is kept turned off, to thereby restrain generation of switching noise and vibration.
It is s further object of the present invention to provide a rotational speed control circuit for a motor which is capable of reducing a loss generated in an excitation current control semiconductor switch during high-speed rotation of the motor and restraining generation of switching noise and vibration during low-speed rotation of the motor without increasing an excitation current while being simplified in structure.
It is still another object of the present invention to provide a rotational speed control circuit for a motor which is capable of readily judging a period of time during which the motor is rotated at a low speed.
It is yet another object of the present invention to provide a rotational speed control circuit for a motor which is capable of protecting a transistor switch used as an excitation current control semiconductor switch from counter electromotive force induced across excitation windings.
In accordance with the present invention, a rotational speed control circuit for controlling a motor including excitation windings of two or more phases is provided. The rotational speed control circuit includes two or more excitation current control semiconductor switches connected to the excitation windings and constructed so as to be kept turned on to permit an excitation current to be flowed through the excitation windings during a period of time for which a control signal is inputted to control terminals thereof, respectively, and a control signal generation circuit for outputting the control signal of which a duty ratio is varied depending on a speed command to the excitation current control semiconductor switches.
The rotational speed control circuit also includes a turn-off period extension circuit arranged with respect to the excitation current control semiconductor switches. The turn-off period extension circuit is constructed so as to permit a turnoff period from a state at which the control signal is ready to turn off the excitation current control semiconductor switches to a state at which the excitation current control semiconductor switches are actually turned off to be extended to a degree sufficient to reduce generation of switching noise during a low-speed period for which the rotational speed of the motor is kept below a predetermined level.
The xe2x80x9cpredetermined levelxe2x80x9d is optionally determined depending on applications of the motor. For example, when the motor is constituted by a fan motor adapted to be incorporated in an OA equipment, the predetermined level may be defined to be about 50 to 90% of a maximum rotational speed of the motor. Extension of the turn-off period of the excitation current control semiconductor switches only during the low-speed period for which a rotational speed of the motor is kept at a low level permits a rate of change (dI/dt) of an excitation current at the time when the semiconductor switches are turned on to be reduced, leading to a reduction in switching noise or electromagnetic sound, resulting in generation of vibration being restricted. Also, extension of the turn-off period is not carried out when rotation of the motor is kept at a high speed, to thereby eliminate disadvantages such as an increase in loss of the semiconductor switches, an increase in generation of heat from the motor due to an excessive increase in excitation current, and the like.
The turn-off period extension circuit may be constructed as desired. For example, the turn-off period extension circuit may include a low-speed period judging circuit for judging the low-speed period and two or more turn-off period extension signal generation circuits arranged with respect to the excitation current control semiconductor switches, respectively. The turn-off period extension signal generation circuits each are constructed so as to feed the control terminal with a turn-off period extension signal after the control signal is ready to turn off the excitation current control semiconductor switch while the low-speed period judging circuit judges the low-speed period. Such construction permits extension of the turn-off period to be positively attained while ensuring satisfactory controllability and prevents occurrence of loss in high-speed rotation of the motor and an increase in generation of heat therefrom.
The turn-off period extension signal generation circuit preferably includes a capacitor which is charged when the excitation current control semiconductor switches are kept turned on during the low-speed period and which carries out discharge through the control terminal when the control signal is ready to turn off the excitation current semiconductor switches during the low-speed period. Such construction permits the turn-off period extension signal to be generated due to discharge of a capacitor decreased in capacity, so that extension of the turn-off period may be attained while simplifying the structure.
A circuit for controlling charge/discharge of the capacitor may be constructed as desired. For example, the circuit may include a capacitor charge/discharge control switch circuit connected in series to the capacitor and controlled by an output of the low-speed period judging circuit in a manner to be turned on during the low-speed period and turned off during the remaining period. Such arrangement of the capacitor charge discharge control switch permits the capacitor to be separated from the circuit during high-speed rotation of the motor, so that the capacitor does not cause any problem during the high-speed rotation even when it is used for extension of the turn-off period.
The low-speed period judging circuit may be constructed so as to directly detect a rotational speed of the motor to detect the low-speed period. Alternatively, it may be constructed so as to indirectly detect the rotational speed to detect the low-speed period. For example, the rotational speed may be indirectly detected on the basis of the speed command. In this instance, the low-speed period judging circuit may be constructed so as to judge the low-speed period based on the rotational speed commanded by the speed command. Also, in this instance, the low-speed period judging circuit may be constructed so as to carry out comparison between the speed command and a reference level, to thereby judge the low-speed period depending on whether the speed command exceeds the reference level. When the speed command is constituted of a voltage signal, the low-speed period judging circuit may judge the low-speed period depending on whether a voltage level of the voltage signal exceeds a predetermined voltage level. This permits judgment of the low-speed period to be facilitated. The low-speed period judging circuit may be constructed by connecting a plurality of diodes to each other in series.
Also, this permits the structure to be highly simplified, because a voltage level is determined depending on the number of diodes or voltage drop across the diodes. In order to increase a range of setting of the voltage level, the low-speed period judging circuit may be constituted by a comparison circuit for comparing the speed command and voltage level with each other.
Generation of the speed command depends on a mode of control of the motor. In a fan motor used for cooling electronic components or an electronic equipment generating heat, the speed command may be varied depending on a variation in ambient temperature. For example, when an ambient temperature is elevated, a rotational speed of the motor may be increased. When the temperature is reduced, the motor is reduced in rotational speed.
The present invention may be applied to a rotational speed control circuit for controlling a fan motor including excitation windings of two or more phases. In this instance, the rotational speed control circuit includes two or more excitation current control transistor switches connected to the excitation windings and constructed so as to be kept turned on to permit an excitation current to be flowed through the excitation windings during a period of time for which a control signal is inputted to control terminals thereof, respectively, a control signal generation circuit for outputting the control signal of which a duty ratio is varied depending on a speed command to the excitation current control transistor switches, and a turn-off period judging circuit including a low-speed period judging circuit for judging a low-speed period and two or more turn-off period extension signal generation circuits respectively arranged with respect to the excitation current control semiconductor switches. The turn-off period extension signal generation circuits each are constructed so as to extend a turn-off period from a state at which the control signal is ready to turn off the excitation current control transistor switches to a state at which the excitation current control transistor switches are actually turned off.